partial revert of ceae3f1f3
SignalMessaging doesn't really need to know much about the SignalCall, and
since SignalCall now knows about WebRTC, keeping SignalCall in SignalMessaging
would mean moving *all* of WebRTC into SignalMessaging.
Instead, we move SignalCall back to SignalMessaging, and only expose a small
protocol thereof to SignalMessaging.
3a5455410e
